Magnetic Field Calc

[Mayimbe]

Hello,

Have anyone here have tried to calculate the magnetic field produced by an OHL using the procedure shown in the EPRI red book? If so, have you ever tried to get the response if there are earth return wires below the conductors (not shieldwire).

Thanks.

 

[Julius Right]

I think you are referring to ch. 7.4.3 "Calculation of Magnetic Field from Power Lines Using Simple Equations" or may be to the Table 7.4-1 "Field Component Due to Earth Currents."
However, if you would be more specific a concrete answer could be possible, I guess.

 

[mivey]

For a transmission line I suspect the impact would be small due to the distance from the phase conductors. In fact, I put in a return conductor close to the surface and it dropped the net field ever so slightly.

But is that what you meant? Are you trying to modify the model of a deep image? If you assume a good magnetic conductor at the surface then the field is opposite and cancels at the surface.

Please give more detail about your question and what you want to do.

 

[mivey]

Are you trying to modify the model of a deep image? If you assume a good magnetic conductor at the surface then the field is opposite and cancels at the surface.

Well, close to that anyway. It will cancel between the image and the conductor and if the image is at the surface then half way between.

 

[mivey]

Another scenario thought: Single-phase lines.

If you mean a SWER, the magnetic field near the ground will be large.

If you put a return near the phase conductor, the magnetic field near the ground will be minimal.

If you put a return conductor in the earth (say 4 ft), the magnetic field near the ground will be small.

Again, more info about what you seek will provide better answers.

 

[resistance]

Lost

 

[Mayimbe]

Hello,

Well, there's an example in that Red book showing a 800 kV flat configuration OHL with 2000 A flowing through their conductors. I simulated that in a calculation software and obtained the same results, with a corridor of 100 m.

I went on and started to simulate different line configurations and current to see the response of the magnetic field below the line up to 100 m from the center of the line.

I did it because there is a tower provider stating that with his towers and a certain configuration he can obtained a certain MF field value. Problem is, they say in one of their leaflets that they have two earth return conductors below the phases (its a double circuit config), and as you mentioned those conductors help to reduce the magnetic field at ground level below the line.

So, I just wanted to know if anyone have crossed to that problem before, of seeing the differences between having those conductors or not, and a reference in how to calculate those Magnetic fields.

Thanks

 

[mivey]

Earth return conductor is not a clear term and is used more than one way. Could be dirt, or dirt enhanced with metal (counterpoise). I don't consider a strung conductor to be an earth return even though it is called that by some. You need to clarify.

Still not clear about what type system and conditions you are investigating. AC or DC? Single-phase or three-phase AC, monopole or bipole DC? Transmission, distribution, or transmission with distribution underbuild? Counterpoise system? Delta or wye configuration? What type grounding? Voltage level? Telecom an issue? Right-of-way issue? Fault issue?

Adding a metallic return conductor for SWER or HVDC is a big factor in reducing the magnetic field. Not so much for a balanced AC transmission or distribution under normal load. More impact with fault conditions on normally balanced AC systems.

As for the calculation, just sum the field vectors from each contributor.

 

[Mayimbe]

Earth return conductor is not a clear term and is used more than one way. Could be dirt, or dirt enhanced with metal (counterpoise). I don't consider a strung conductor to be an earth return even though it is called that by some. You need to clarify.

Still not clear about what type system and conditions you are investigating. AC or DC? Single-phase or three-phase AC, monopole or bipole DC? Transmission, distribution, or transmission with distribution underbuild? Counterpoise system? Delta or wye configuration? What type grounding? Voltage level? Telecom an issue? Right-of-way issue? Fault issue?

Adding a metallic return conductor for SWER or HVDC is a big factor in reducing the magnetic field. Not so much for a balanced AC transmission or distribution under normal load. More impact with fault conditions on normally balanced AC systems.

As for the calculation, just sum the field vectors from each contributor.

o
O O
O O
O O
. .


______

where,
o = shieldwire, ground wire or earth wire
O = conductors with 2000 A each. AC
. = earth return conductor

__ = ground

Put dimensions as you like, no more information needed as per EPRI red book MF calculation. Problem is to represent the earth return conductor (.) in that calculation, Dont know how to do it, suggestions?

 

[Mayimbe]

Adding a metallic return conductor for SWER or HVDC is a big factor in reducing the magnetic field. Not so much for a balanced AC transmission or distribution under normal load. More impact with fault conditions on normally balanced AC systems.

any reference?

 

[mivey]

o
O O
O O
O O
. .


______

where,
o = shieldwire, ground wire or earth wire
O = conductors with 2000 A each. AC
. = earth return conductor

__ = ground

Put dimensions as you like, no more information needed as per EPRI red book MF calculation. Problem is to represent the earth return conductor (.) in that calculation, Dont know how to do it, suggestions?

See section 7.9. The induced currents will reduce the magnetic field slightly in the right of way under the line but outside the right of way I would not expect much. It might vary some with the phase arrangement on the double circuit.

 

[mivey]

any reference?

I guess I could find some but just consider:
Large earth current = large field
Large return conductor current = large canceling field
Line-ground faults = increased earth currents
Balanced AC = minimal earth current

 

[Sahib]

Consider a single phase-neutral system in which some current is flowing and no return earth conductor. If you put a clamp meter around the phase conductor or neutral conductor, you will measure that current. But if you put the clamp meter around both of the conductors, you will measure zero current and so zero magnetic field. The same kind of phenomenon occurs in case of three phase supply also.

 

[Mayimbe]

I guess I could find some but just consider:
Large earth current = large field
Large return conductor current = large canceling field
Line-ground faults = increased earth currents
Balanced AC = minimal earth current

Yep, I can see that.

Problem is that the client is specifying a magnetic field value below the line that is not convincing with the info given.
It is a balanced system, but again, a completely ideal balanced system doesn't exist, hence the need to represent those earth return conductors in the calculations.

Thanks anyway.

 

[mivey]

Problem is that the client is specifying a magnetic field value below the line that is not convincing with the info given.
It is a balanced system, but again, a completely ideal balanced system doesn't exist, hence the need to represent those earth return conductors in the calculations.

If the transmission line is delta, the earth return has mostly induced currents as you would calculate using section 7.9. If the system is wye, like with distribution, we try to keep it balanced so it is similar to a delta system. For systems with unbalanced load current in the return, I would think you would get more bang for your buck by modifying the phase arrangements. Modeling is done using the self and mutual impedances as described in the red book.

Mitigating the magnetic fields by phase arrangement is also done in the delta system. Of interest is if you are trying to reduce the near (in ROW) or far field (outside ROW). Of the magnetic field mitigation techniques discussed in the EPRI red book (there is a whole section), I seem to recall shielding is one of the least useful (there is a section on looping but that is relatively expensive IIRC). The shielding might be OK for mitigating the electric field but not so much for the magnetic field.

You can make these calcs by hand but honestly it is much better in software, especially with the matrices involved. Plus software gives animations, graphs, etc. pretty painlessly. I'll be glad to help you if you like. It is pretty simple to set up the tower and conductor configuration, apply the loads, set a metering point and get a result to validate or invalidate what you have been told.

 

[Mayimbe]

If the transmission line is delta, the earth return has mostly induced currents as you would calculate using section 7.9...

Thanks for that, I have just realized that you were referring to the same Red book BUT 2005 ed. I was looking at the 1982 ed. instead.

 

[mivey]

Thanks for that, I have just realized that you were referring to the same Red book BUT 2005 ed. I was looking at the 1982 ed. instead.

Yes. Sorry about that. That would be chapter 8 in the second edition.

Check out the chapter's subsection 17 on field mitigation techniques.

 

[mivey]

Yes. Sorry about that. That would be chapter 8 in the second edition.

Check out the chapter's subsection 17 on field mitigation techniques.

Section 8.17 in the 2nd edition was a new section dealing with electric field mitigation. The magnetic field mitigation was mentioned in chapter 13 referring to a publication dealing with pipeline interference. An interesting read if you want to read over 600 pages.

Magnetic mitigation was added in the red book 3rd edition in Section 7.17 and gets to the point.

Due to the limited effect of the underbuilt ground wires are you sure they were not talking about electric field reduction instead of magnetic field reduction?

 

[Mayimbe]

Due to the limited effect of the underbuilt ground wires are you sure they were not talking about electric field reduction instead of magnetic field reduction?

Yes, completely sure. We need to achieve 0.4 uT (microTesla) at 50m from the center of the line.

It is kind of sad that they don't show the calculations anymore, instead they offer a sort of software now.

Anyway, I think I can figure it out from here.

Thanks.

 

[beanland]

Have used Finite Element Analysis on similar

Have used Finite Element Analysis on similar

If the "shield" wires are multipoint grounded, current is induced. That current tends to reduce the magnetic fields "under" the shield wire. Phase configuration makes a huge difference in magnetic fields. FEA is a great tool for assessing electric and magnetic field strength. EPRI red book provided software for calculating fields, it works quite well.